The size of your pupils fluctuates dozens of time during the day in response to changing light. Assuming you're not exercising in the dark, some other factor is at play if your pupils are dilated during a workout. Dilated pupils should be low on your list of worries unless they're accompanied by other symptoms, but taking note of your pupils can give you clues about whether you're exercising as safely as possible.
Pupil Dilation Basics
The round, dark pupils inside the colored iris of the eye are openings that get larger, or dilate, to allow more light into the eye when necessary. Your pupils dilate when you're in a dark place, then get smaller once exposed to light. The pupils will also dilate when you're under a great deal of stress; your body takes stress as a sign of a threat, so it's biologically wired to take in more light so you can deal with the danger. Enlarged pupils can also result from heat stress or drug use, they can be a sign of a medical problem or they can indicate that you are attracted to the person you're looking at.
If your pupils dilate after a prolonged workout on a hot day, it's possible you're suffering from heat exhaustion or heat stroke. These conditions occur when your body is overheated and dehydrated. Dilated pupils alone aren't a sign you have heat exhaustion, but they may go along with headache, nausea, cramps and extreme thirst. If you develop symptoms of heat exhaustion, you must stop exercising and start drinking water and electrolyte replacement drinks right away. If you leave heat exhaustion untreated, it can progress to heat stroke. The symptoms of this extremely dangerous condition can include hot, pale skin, rapid pulse and pupils that not only dilate, but stay dilated when exposed to light. Call for emergency medical help if you suspect you have heat stroke.
Since dilated pupils are your body's natural response to stress and danger, and exercise can help you mitigate stress, you aren't likely to have dilated pupils during a yoga class or jog around the block. However, if your workout is a high-stress one, your pupils may dilate because your body is under pressure. Climbing a rock wall or taking a boot camp-style exercise class, complete with screaming instructor, may send a "fight or flight" signal to your brain. As long as you stretch thoroughly, drink plenty of water and stop exercising if you feel pain or sickness, you may continue with your intense workout, but if you feel anxious during exercising, switch to something more low-key.
A concussion, or brain injury, is usually temporary and commonly accompanied by nausea and dizziness. Dilated pupils can be a sign that your concussion is serious, so if you notice this symptom after knocking heads with someone or getting hit in the head with a ball, get medical help right away. Certain drugs, such as marijuana, cocaine and amphetamines, cause dilated pupils. Never exercise if you're under the effects of drugs. They'll dull your senses and reaction times and increase your risk of injury.